Circular (Iowa State College. Agricultural Iowa Agricultural and Home Economics Experiment Station) Experiment Station Publications

4-1913 Green Manuring and Soil Fertility P. E. Brown Iowa State College

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Recommended Citation Brown, P. E., "Green Manuring and Soil Fertility" (1913). Circular (Iowa State College. Agricultural Experiment Station). Paper 10. http://lib.dr.iastate.edu/iaes_circulars/1

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Abstract Green manuring puts humus into the soil and that makes it an important farm practice, because humus is absolutely essential for the best growth of crops. Humus may be added in another way, by applying barnyard manure, but that method alone will not keep the soil from losing fertility from year to year. Even though it is saved with the utmost care, the manure produced on the farm will not return to the soil all the fertility removed by the crops. Circular No. 9, "Farm Manures," Iowa Agricultural Experiment Station, shows furthermore, that there are certain unavoidable losses in the storing of manure and that they may involve not only the valuable mineral constituents, but also the organic matter. As agriculture is now generally practiced in Iowa and elsewhere, the soil is being gradually worn out.

It is clear, therefore, that some other means must be employed to keep up the humus content of soils and this may be accomplished by turning under green manure crops.

Keywords Agronomy, Soils

Disciplines Agricultural Science | Agriculture | Agronomy and Crop Sciences | Soil Science

This article is available at Iowa State University Digital Repository: http://lib.dr.iastate.edu/iaes_circulars/1 Circular No. I 0 April, 1913

GREEN MANURING AND SOIL FERTILITY

AGRICULTURAL EXPERIMENT STATION IOWA STATE COLLEGE OF AGRICULTURE AND THE MECHANIC ARTS

AGRONOMY SECTION Soils

Ames Iowa OFFICERS AND STAFF

IOWA AGRICULTURAL EXPERIMENT STATION

STATE BOARD OF EDUCATION

Hon. J. H. Trewln, Cedar Rapids. Hon. A. B. Funk, Spirit Lake. Hon. George T. Baker, Davenport. Hon. Charles R.. Brenton, Dallas Center. Hon. E. P. Schoentgen, Council Blutl's. Hon. Parker K. Holbrook, Onawa. Hon. D. D. Murphy, Elkader. Hon. Roger Leavitt. Cedar Falls. Hon. Henry M. Eicher, Washington.

OFFICERS

Hon. J. H. Trewln, Cedar Rapids •.•....••...•.•.••.•.•••••••••• President Hon. D. A. Emery, Ottumwa •.•.•.•..•.•••.•••...•...••••••.••• Secretary

FINANCE COMJ.\IlTTEE

Hon. W. R. Boyd, President, Cedar Rapids. Hon. Thos. Lambert, Sabula. Hon. D. A. Emery, Secretary, Ottumwa.

AGRICULTURAL EXPERIMENT STATION STAFF

Raymond A. Pearson, 1\t. S. A.. LL. D., President. C. F. Curtiss, l\1. S. A., D. S., Director. W. H. Stevenson, A. B., B. S. A., VIce-Director. J B. Davidson, B. S., .l\1. S. A., Chief In Agricultural Engineering. W. H. Stevenson, A. B., B. S. A., Chief In Agronomy. H. D. Hughes, M. S., Chief In Farm Crops. L. C. Burnett, M. S. A., Assistant Chief In Cereal Breeding. P. E. Brown, B. S., A. 1\1., Ph. D., Assistant Chief In Soli Bacteriology. John Buchanan, B. S., Superintendent of Co-operative Experiments. Charles R. Forest, Field Superintendent. · E. H. Kellogg, B. S., Assistant In Soli Chemistry. Robt. Snyder, B. S., Assistant In Soli Chemistry. W. H. Pew, B. S. A., Chief In Animal Husbandry. H. H. Klldee, B. S. A., Assistant Chief In Dairy Husbandry. J. M. Evvard, M. S., Assistant Chief In Animal Husbandry. Geo. 1\1. Turpin, B. S., Assistant Chief In Poultry Husbandry. D. B. Adams, Herdsman. · R. E. Buchanan, M. S., Ph. D., Chief In Bacteriology; Associate In Dairy and Soli Bacteriology. L. H. Pammel, B. Agr., 1\1. S., Ph. D., Chief In Botany. Charlotte 1\1. King, Assistant Chief In Botany. Harriette Kellogg, Assistant In Botany. A. W. Dox, B. S., A. l\1., Ph. D., Chief In Chemistry. R.E. Neidig, M. S., Assistant In Chemistry. W. G. Gaessler, B. S., Assistant In Chemistry. S. C. Guernsey, M. S., Assistant In Chemistry. W. Eugene Ruth, 1\1, S., Assistant In Chemistry. J. C. Reese, B. S., Assistant In Chemistry. M. Mortensen, B. S. A., Chief In Dairying. B. W. Hammer, B. S. A., Assistant Chief In Dairy Bacteriology. H. E. Summers, B. S., Chief In Entomology. R. L. Webster, A. B., Assistant Chief In Entomology. S. A. Beach, B. S. A., l\1. S., Chief In Horticulture and Forestry. Laurenz Greene, B. S. A., 1\1. S. A., Assistant Chief In Horticulture. G. B. MacDonald, B. S. F., Assistant Chief In Forestry. J. H. Allison, Assistant In Plant Introduction. T. J. Maney, Assistant In Horticulture. C. H. Stange, D. V. l\1., Chief In Veterinary Medicine, F. W. Beckman. Ph. B.,- Bulletin Editor. F. c. Colburn, Photogmpher. GREEN MANURING AND SOIL FERTILITY

By P. E. Brown

Green manuring puts humus into the soil and that makes it an important fabn practice, because humus is absolutely essential for the best growth of crops. Humus may be added in another way, by applying barnyard manure, but that method alone will not keep the soil from losing fertility from year to year. Even though it is saved with the utmost care, the manure pro· duced on the farm will not return to the soil all the fertility 'removed by the crops. Circular No. 9, ''Farm Manures,'' Iowa Agricultural Experi· ment Station, shows furthermore, that there are cea-tain unavoidable losses in the storing of manure and that they may in\·olve not only the valuable mineral constituents, but also the organic matter. As agriculture is now generally practiced in Iowa as well as elsewhere, the soil is being grad· ually worn out. It is clear, therefore, that some other means must be employed to keep up the humus content of soils and this may be accomplished by turning under green manure crops.· THE J'ALUE OF GREEN MANURES Any crops grown and plowed under green to improve the condition of the soH are called green manure crors. When such crops are used in addition to a regular rotation and occupy the land for only a part of the season, they are known as '' co\·er '' crops or • • catch'' crops. The value of green manures as a means of impro,·ing soil conditions has been known for eenturies and in the light of past experience it is somewhat surprising that their use is not more common now. Green manur:ng is not enough in itself, of course, to keep a soil from wearing out. There are other practices that must be followed also. Crop rotation must be practiced. The amount of moistu:e in the soil must be regulated. Proper tillage must be employed. Steps must be taken to keep soils from beeoming acid. The supply of the mineral plant foods, es­ pecially nitrogen, phosphorus and potassium, must be kept up. This may be done in the ease of nitrogen by growing legumes that take nitrogen from the air. Potassium is not likely to be deficient in Iowa soils for centuries. Phosphorus must be applied as it is needed. These faetora a:-o all imporl!lnt in keeping a soil fertile, but not more important than green manuring. In general, it may be said that green manures improve poor soil becau&e of their action on the physical, chemical, and bacteriological conditions r-er· taining to the soil.

THE PHYSICAL EFFECTS OF GREEN MANURES The charade::- of a soil very largely determines the physical ell"eets of a green manure upon it. If light, and sandy, it will be all"eetcd quite dif· ferently than if heavy and clayey. In general, bowe'l'"er, green manures in· fluence the moisture conditions, the temperature, the aeration, and the • 4

Sol· Beans on the Iowa Agricultural Experiment Station Plot. sourness of ooils. Through these physical effects, the chemical and bnc· teriological conditions are consida:ably altered. MOISTURE On light, sandy soils, green m:mure pre\·ents rapid drying out, which 90 often occurs, by cutting down the losses of moisture due to evaporation and percolation. Green manures make the soil more compact by uniting the fine particles into Jargct- masses, and the humus or organic matter, itself acts like a. sponge in holding water. Thus by the use of green manures, sandy soils, otherwie infertile, may be made to hold enough moisture for crop production. Green manures ha\·e the opposite effect on heavy clay soils. The organic matter opens them up, admitting air, and p:-eventing extreme moisture con· ditions. It also makes them less stiff and more crumbly when dry. Thus not only is the n:techanical <'onditions of the soil made better, but the chemical and bacteriological conditions are also much improved. Green manuring may exert certain mechanical effects on both hea\-y and light soils in particular cases, as in protecting hilly soils against washing and sandy soil against blowing. TEJCPERATURE Tho presence of humus produced from decomposition of green manures tends to lessen extremes of temperature in the soil. Thus it will make it cooler in summer and warmer in winter; cooler in the daytime and wanner at night. Extremely high temperatures in sandy soils in summer are pre\·ented and the Joss of valuable constituents wl1ich may result from such high temper· atures is restricted. Vcry low temperatures in "cold" clay soils are like· wise pre\·entcd and the trnnsformation of mineral plant. food into available form which might be restricted considerably is permitted to continue at. a normal rate. · AERATION Since green manures open up clay soils and make sandy soils more com· pact by adding organic matter, or humus, they ha,·e an important influence on the amount of air p:-esent in the soil. Humus acts beneficially on light, open soils in which too much air is apt to be present and in which destruc· 5 tive fermentath·e processes may occur with loss of valuable constituents. It restricts the entrance of air and reduces the destructive processes and consequently the losses. In heavy, tight, elay soils, on the other hand, fermentative }l'rocesses are apt to be too slo\v. In these the organic matter from green manures opens up the soil, admits air, and permits the more rapid production of plant food. The subsoil is also opened up to a eon· siderable extent and the unavailable plant food tl1ere is brought up and made available. REACTION OR "SOURNESS" The turning under of a crop of green manure adds a large amount of organic matter to the soil. In its dceomposition, acids may be produced in considerable amounts and the soil become acid or "sour" in reaction. As is well known, sour soils are somewhat infertile, so when green manures S:"e employed the reaction of the soil should be carefully tested. If it is acid, limestone should be applied in amounts depending on how "sour" the soil is and the crop to be grown.

THE CHEMICAL EFFECTS OF GREEN MANURES Although the main effects produced in a soil by adding the organic mat· ter contained in a green manure crop are physical, the c!Jemical and bae· teriological conditions are also influenced to a considerable extent, as well as the plant food production which is governed by these conditions. THE CONSERVATION OF PLANT FOOD Humus not only absorbs and xet:1.ins moisture, but it also absorbs plant food which might otherwise be lost. It retains nitrogen in the form of ammonia, and it absorbs soluble potass:um compounds which might be leached out of the soil and retains them for plant nourishment. Nitrogen IDIJ:Y be lost from the soil not only by passing into the air as ammonia(. bp.t,)also by the leaching of soluble nitrntes Into the drainage water and~by the escape of nitrogen gas from nitrates into the air. There is ,·ery little plant food in the drainage water from soils uncler crops, while on fallow soils as much as 40 lbs. of nitrates may be lost annually. Hence the ·presence of green manure crops on the soil at times when it would otherwise be bare prevents the loss of nitrates as these are taken up by the plants and so stored in the soil for the suceeeding crop. Furthermore, the period of most acth·e nitrification in soils, that is, the time at which the greatest production of nitrntCl!l ooours, is during July and Augut and consequently if the soil is left bare following a small g=ain crop all the nitrates produced in the soil may be lost in the draina~e water. It has been estimated that this loss of nitrogen from soils on which wheat is grown continuously amounts to 4 to 6 lbs. of nitrogen .for each pound used by the crop. The late summer and early fall is not. only the peri()(l of greatest ae· tivity of the nitrifying bacte;ia, but also of many other species. Consc· quently, many other 110luble plant food constituents are produced which might be lost by leaching .if no crop were present to absorb and retain them. THE CONCENTRATION OF PLANT FOOD The green manure crops, espceially the legumes. may act ns collectors of plant food, building thcmseh·es up by drn\ving their nourishment f~m the lower soil layers. When such a crop is plowed under, all the food con· tained in it is stored near the surface for the benefit. of succeeding crops. The roots of alfalfa, for instance, may elltend from 12 to 30 feet down· wards and much of its food is thErCby brought up from the subsoil. This food is stored in the crop itself and in the large tap roots of the plants •6 near the surface. When the plants are turned under, this food is retained ncar the surface for the use of succeeding crops. THE INCREASE IN AVAILABLE PLANT FOOD When a green man'lbl"e crop is turned , under in a soil, moisture conditions being favorable, many species of bacteria begin to a~t. on its mass of or­ ganic matter. They change the proteins, the complex, insoluble compounds which contain the nittogen, into soluble proteins, then into amino acids, then into ammonia, and finally into nitrates, in which form the nitrogen is taken up by plants. The non-nitrogenous matter in the green manure is also acted upon by various groups of bacteria which decompose the carbohydrate and cellulose nmterials more or less completely into carbon dioxide, organic acide, etc. This carbon dioxide and the organic acids act on the insoluble phosphorus and potassium compounds in the soil and change them into soluble, available forms. Thus the destruction of a green manure crop in the soil not only transforms the nil\rogen in the crop into a form that plants can use but it also makes the plant food in the soil soluble. THE INCREASE IN NITROGEN .When leguminous crops arc used as green manures there may be an in· ·crease in the nitrogen content of the soils. Legumes are independent of the nitrogen in the soil if their roots have so-called nodules, or swellings, which contain bacteria. These bacteria take the nitrogen from the atmosphere and supply it. to the plants and thus there is no draft on the nitrogen in the soil. If the entire crop is turned under there is an addition of nitrogen to just the extent to which the ni· trogen from the air is stored in the plants. For this increase of nitrogen in the soil to occur, the legumes must be inoculated; that is, if the proper bacteria are not already present in the soil, they must be put there. This is called ''inoculation'' and may be accomplished by the use of inoculated soil or by pure commercial cultures of the proper organisms. Further de· tails concerning the inoculation of legumes may be obtained in Circu!Oil'l No. 8 of the Iowa Agricultural Experiment Station. · Green manures also bring about an increase in the nitrogen content of soils by encouraging the acth·ities of the non-symbiotic, or free-living bac· teria, supplying them \\;th energy so they can fix nitrogen from the at· mosphere in the soil. TH BACTERIOLOGICAL EFFECTS OF GREEN MANURES The bacteriological etrects of green manuring depend so closely on the physical and chemical changes caused that it may be said that they are the direct result of those changes. The variations in moisture content, temper­ ature, aeration, and reaction or "sourness" of soils caused by green manur· ing atrect the bacteria \'cry considerably. Bacteria require water for their growth just as the higher plants do and they are similarly atreeted if tho~e is too little or too much moisture. If the water content of a soil is l'ery higl1, beneficial bnderia will be checked in their growth and the production of plant food, which is the main func· tion of bacteria in the soil, will be reduced. So also if the soil becomes too dry, bncteria will be reduced in numbers and efficiency. Temperature has a cErtain etrect on bacteria, some species growing best at moderately high temperatures and others preferring lower ones, and the activities of many beneficial species are reduced by extremes of temperature. The amount of air present will determine to a lnrge extent the kind of bactldal acti\'ity going on in the soil. If abundance of air is present then the aerobes, the bacteria needing air for their growth, will flourish and a large production of available plant food may occur. If the entrance of air 7

is restricted then the anaerobes, those bacteria gr.-owing without air' will prevail and the production of available food is restricted. The sourne88 of the soil also has an important influence on its bacterial activities. Bac· teria genernlly prefer a soil which is not sour and if acids accumulate to any extent the numbers will grow considerably lc88 and the production of avail· able plant food will be correspondingly decreased. Thus it is e,·ident that as the usc of green manures improves the physical condition of soils along tl•ese lines, beneficial bacterial activities are en· couraged and like•,;se the production of plant food. In other words, im· proved physical conditions lead to greater bactc:-ial action. Those bacteria which attack proteins and transforms them through the various stages already enumerated, into nitrates are encouraged. Those organisms also which at· tack purely non-nitrogenous compounds are more active; greater amounts of organic acids and carbon dioxide are formed and the transformation of insoluble phosphorus and potassium compounds into soluble form is increased. "'e find therefore, that tho chemical effects of the use of green manures which ha•·c already been discussed a.:e the result of bacterial action. Avail· able plant food in the soil is increased through the action of bacteria and the increase in nitrogen content by the use of legumes is the direct re· suit of bacterial inoculation. A further increase in nitrogen in the soil may be brought about by the no!J·symbiotic nitrogen-fixing organisms which a:-e .encouraged to fix more nitrogen in the soil by the energy supplied in the organic matter of the green manure.

THE CROPS WHICH MAY BE USED FOR GREEN MANURES There are two classes of crops which may be used as green manures, '• ni· trogen consumers" and "nitrogen gatherers." The "nitrogen consumers" p!'event the lo88 of \"aluable constituents and impro,·e the physical and ab· sorptive power of the soil but add no nitrogen directly and the "nitrogen gatherers" ha,·e the same effects but in addition they put nitrogen into

Cowpeu on the Iowa Acricullun•l Experiment Station plota. 8 tho soil. Among tl•e first g;-oup we find the cereals, the grasses, buck· wheat, turnips, rape, oh•. Because of their time of growth and period of rapid !levelopment it is often desirable to grow them for green manures when the chief pu.rpose is to save plant food and it is of only minor im· porta nee to add nitrogen 1 to the soil. The second group includes all the legumes, such as the clovers, pens, beans, vetches, alfalfa, lupines, etc. The great advantage from the usc of legumes for green manures is that they may be grown absolutely independent of the nitrogen in the soil. The ni· trogen which a legume takes from the air is ndded to the soil when it is plowed under and thnt increnses considembly the nit.--ogen content of tl1e soil. As nit.rogen ia the essential plnnt food .most apt to be Jacking in soils, this adding it by plowing under legumes is quite an importnnt con· sideration. NITROGEN CONSUMING CROPS Rye has many ndmntages as a green manure crop. It may be seeded late and will make a good growth during tho Into fall. It withstands cold winters well and makes an early spring growth and consequently may be plowed under for early crops. Tho cost of seeding rye is small and all farmers are familiar with tl1e method of preparing for the crop and hand· ling it. It absorbs the plant food which ptight be lost in the fall and it adds a large amount of humus to the soil, but it docs not add nitrogen. The chief value nttacbed to rye is its bardiness in 11ithstanding cold winters and its early spring growth. Wheat is less satisfactory than rye as s green manure for the reason tbnt, nlthough it is somewhat tbe same, it is not so hardy and does not make as early a spring growth. Furthermore, much more care must be taken in preparing the soil for wl1eat tlmn for rye .and the seed is more expensh·e. The value of wheat as a grnin crop is too great to warrant its extensi\·e use ns a green manure crop. Buckwheat makes a mlunble green manure on poor lands. Its season of growth is during .Tuly and August and it is valuable therefore in keeping the soil covered during the hot weather of those months, preventing the less of plant food which would othcm·iso occur. It also adds a eonsiderable amount of humus to the soil. Turnips are frequently used for green manures because of their rapid - appropriation of plant food in cool weather. They make a large accumu· lation of organic matter in the late fall and if deep rooted varieties are employed, much plant food is brought up from the lower soil layers and stored in the crop for the benefit of succeeding erors. Dwarf Essex Rape is occas'onnlly employed to serve as a eover crop during tl1e winter, as it may be grown in tiiC late fall. It possesses no particular ndvnntages over rye or the other nitrogen consuming crops. NITROGEN GATHERING CROPS Clovers nrc the most common and best known legumes and the good ef­ fects of their use as green manures is a matter of common knowledge_ Red Clover is perhaps the crop most generally used for this purpose. Its ndmntnges nrc very largely due to the extensh·e root system which not only opens up the soil but also enables the plant to draw its food from the

Sweet CloYer 10

Crimson clover has beon used with considerable success in the. eastern states but it is affected by sc,·cre weather and is particularly sensitive to drought. It is essentially a cool weather plant, grows late in the fall and mnkes an early spring growth. Great care must be observed in pre· paring for th!s crop. Because of the mulch during the winter, prevent• ing losses from the soil. · Cowpeas nrc particularly adapted for wann climates but some varieties do well in cool climates also. They make a rapid and large growth during the hot summer months, when it is very des'rable to have the soil co\·ered. They possess considerable ability in the fixation- of nitrogen and when well inoculated may add a largo amount of this \'alunble constituent to the soil. They may be plowed under green in the fall or left on the surface as a mulch during the winter and plowed under in the spring. Soybeans resemble cowpens in many ways but they are more difficult to handle and they nre more sensith·c to cold. They should not be seeded until the soil is wann. While their yields nrc not so heavy as cowpcas, "the plants contain more nitrogen in tbe dry matter. · Vetches, winter and spring varieties, are used for green manures witb considerable success but the seed costs too much to make their usc pay. Tiley are superior to cowpens or soybeans on light soils, and they may also be sown in tbe early spring or in the fall.

THE USE OF GREEN 11/ANURES llany important questions nr;se in the use of green manures. In the first place it 's necessary to determine when green manuring siJould be prac­ tised and whether under the conditions a legume or a non-legume should be chosen. Then there must be !'arcful preparation of the soil, and con­ sidl't'ation must be gi\·en to tbe factors tbnt influence tbe crop. Also, the farm11r must take into account the effect of green manures on succeeding crops and tbe conditions go,·erning the plowing under of the crop. WHEN GREEN M~INURING SHOULD BE PR.A.CTISED Green manurlng should not be used without good reason any more than any other agricultural practice, for e\·en the most commonly beneficial ope:ntion may sometimes injure the soil. Thus while general principles and 11

Cowpe;u In Com directions go\·erning the use of green manures may be gh·en, local soil, climatic and other conditions are of so much importance that definite ad· vice cannot be gh·en without knowledge of these particular conditions. In grain farming, where the produce is sold from the farm, there is a rapid dec:-ca.se in fertility. The soils become poor not only in the essential plant food constituents but also in organic matter, while their physical con· dition becomes unsatisfactory for crop growth. An ample supply of humus is one of the essential requisites for the ma intenance of soil fertility and in grain fanning the only logical way to insure this supply is by the use or green manures. When a leguminous green manu.-e crop is introduced as an addition to the regular rotation, the humus content of the soil is main· tained and besides much nitrogen may be aoats, and clo\·er, green manures may be used in connection with both corn crors if it is dcemed necessary. It should be used in at least one crop. The fact that the soil is kept co\·ered during the winters following the corn crops is an important point in favor of the use of clover crops in such case. In live stock farming, much of the fertility removed from the soil by the crops may be returned by the proper storing and use of the manure produced on the farm, IUISurning of couri!C that all the produce of the farm is fed on the place. Even under the be8t conditions, however, all the fer· 12 tility removed by the crops cannot be returned in the manure and hence green manuring hRS a place also in Jive stock farming. 'Vhere green manures are used twice on a 21'ain farm there would probably be DO need to use them more than once on a stock farm. Here, again, loeal conditions will determine to a large extent the need and f..equency of the need of or· ganic matter in the soil. THE CROPS TO BE EMPLOYED The relative ach-antnges of legumes and non-legumes have been mentioned, the latter ha\·ing the same effects RS the former to a large extent except that non-legumes do not put nitrogen into the soil. Sometimes a non-legume may accomplish just as satisfactory results as a legume at a less expense and in such cRSes there is no question as to which should be employed. Thus rye may sometimes be used as a cover crop on soils following the corn crop with as much beneficial effect as a clove:-. If the soil is rich enough in nitrogen and it is desired only to add organic matter, improve the physical condition of the .soil, and prevent the loss of valuable constituents, then ll non-legume may be cheaper than a legume and just as good. Again, the local conditions, including the apparent needs of the soil, must aid in the selection of a crop for a green manure. The value of sweet clover as a leguminous green mantt:e crop for Iowa soils has already been pointed out and tests of its use liS a me11ns of main· tnining the fertility of the soils of the state cannot be too strongly urged. THE CONDITIONS GOJ'ERNING THE CHOICE OF A CROP • Certain conditions should· be observed in the choice of a crop. Among these are the cost of seed, the suitability of the crop to the particular climate, and the character and need of the soil. The ease with which the crop may be plowed under is anoth~ point of importance and the ability of the plant to choke out weeds is also of interest if -the crop is to. be used for that purpose on soils permitted to remain bare «!!!!ing the late summer and fall. When legumes are grown, the ability of the plant to secure ni~gen from the atmosphere must in addition to the abo\'e then be considered. So:ne legumes seem to ha,·e mor~ power of utilizing atmospheric nitrogen than others. How deep the plant's roots JtO is another point of interest, for the deeper the root system the gteater the feeding district of the plant and the more plant food will be brought up from the deeper soil layers and stored near tbe surface. Furthermore, the deeper the roots go into the subsoil, the more it is opened up, and the greater is the admission of air. Increased aeration is followed by increased bacterial activities and greater plant food production. Finally, in the choice of a green manure crop, its forage value and the quality and quantity of bay or pnsture it may produce must be considered. Thus in some eases it may be more pt-ofitable to use the crop for forage or hay or to pasture it tban to turn it under for a green manure. It must be remembered .that the roots and stubble of legumes make an important ad: dition to the so:!, even although no deci(led gain in nitrogen is accom~lished. The nitrogen content of the soil need not be depleted to any extent by the growth of legup1es if proper inoeulation is po:-formed. If the humus con· tent is not seriously reduced and it is not necessary to make lar~re addi· tiona of organic matter or nitrogen to the soil to bring it to a fertile state, the use of the crop for feeding purposes may be more ]X;'Ofitnble. THE PREPARATION FOR A GREEN llANURE CROP The soil must he put in as good a state for the growth of a green manure crop as for any other eroJl. The seed bed must be just as carefully pre· pared as if the crop were to be ha.-vested. The soil must have the necessary plant food, except that in the ease of legumes no nitrogen is necessary, pro· 13 vided there is proper inoculation. Thus enou!fh phosphon1s and potassium must be present; if not, the crop will be a failure and the expense and trouble of planting will be wasted. The reac~ion or sourness of the soil must be coccfully detennined, particularly in the case of legumes, some of which nrc very sensitive to slightly acid conditions. In such cases ap· plications of ground limestone should be made before seeding to the legume. Where legumes are used it is· essential also that there be inoculation. The proper bacterin must be present so that the plants may take their nitrogen from the air and be independent of the nitrogen supply of the soil. The soil may be inoculated either by the use of soil !'rom o. field where the legume has previously been successfully gro1m or by the use of pure, com· mercia! cultures. The fonner method is still recommended because the latter is not yet certain. Inoculation by using from 300 to 500 lbs. per acre of soil from a we)) inoculated field of the same legume ha1·e never been known to fail. THE EFFECT OF GREEN J.fANURES ON SUCCEEDING CROPS Many expe:iments ha1·e been carried out showing the effects of green nanures, notably legumes, on succeeding crops. While of eourse these ef· fects vary with the crop employed, as well as with the local conditions, they have in most cases been ,·cry striking. For instance, red clover has been shown to increase the succeeding crop of rorn to ,the extent of 20 buhels per acre, and the following crop of oats to the amount of 10 bushels per ac.--e. Potatoes ha1·e shown a 30 bushel increase wl1en grown following red clo1·er as a green manure. Cowpens ha1·e gi1·en similar ineren11es under certain conditions and other legumes have c.:mse(l varying inereat'Cs in sub· sequent crops. Occnsionally some crop may be unfa1·orably affected by the use of green manures. The large amount of organic matter sometimes leads

· Cowpea.• In Oat Stubble 14

Cowperul in Standing Com to the extensive production of nitrates which may cause the increal!ed de· \·clopment of leaves, and delay the ripening of some fruits, or cause the lodging of some grains. With these few rare exceptions, however, green manures have been sl10wn to increase tho yields of subsequent crops to a profitable extent. THE PLOWING UNDER OF GREEN MANURES The time of plowing under green manure crops will be determined by various factors, among which are the season of growth of the plant, the condition and character of the soil, and the weather and other seasonal conditions. The crop should be plowed under in a green condition when it is full of moisture as then the decomposition processes are favored and its destruction takes place more rapidly. If the plants nrc matured, they lm\·e become woody in structure and de· compose very slowly n.nd may cause drying out of the soil by the increased evaporation which they cause. If a heavy green manure crop is plowed un· 15

der when the soil is dry, and in wann, dry weather, the soil may be in· jured by the reduction of the moisture content and succeeding crops may suffer for laek of water. On the other hand, if green manures are turned under in the late fall, the decomposition p;ocesses are \'cry slow and acid conditions may be deYeloped in the soil. In short, the ideal conditions for the decomposition of green manures are abundance of moisture and a moderately high temperature. "'hen fall sown crops are to follow green manures the J!Teen manure should be turned under at least four weeks prior to the planting of the crop and the soil thoroughly harrowed and compacted to insure proper decomposition of the green matter. CONCLUSION

In conclusion, the importance of ~t:-een manurin~t in all systems of penna­ vent agriculture cannot be too stron~tlY emphasized. ln no other way can the humus content, and therefore the fertility, of a soil be as cheaply main· tained. . The manure produced on tl1e )l\•e stock fanu, although it docs constitute a means of returning to the soils of such farms a portion of the fertility :-emo,·ed by crors, is insufficient to keep the soil fertile. In J!Tilin farming, when the produce is sold, there is absolute neees.~ity of some means of re· turning valuable constltuents, for on these f11nns the manures produced are of little significance, representing as they do such a small portion of tile crops grown. It makes no ditferenee the::efore, whether groin fanning or liYe stock farmin~ is practised, no rotation of crops which does not penuit of the introdurtion of some "co\·er ". or "catch" crop should be con· sidered satisfactory. The wearing out of Iowa. soils is no i11le p:ophecy, it is a reality, which is now close unon us. Already some Iowa soils are hej!innin~ to decline in fertility as e\·ideneed by decreasin!! crop yields and the reason :s that proper methods of fannin~ are not practised. El"ery fanner should conside:\ well the t>~~Rential factors influencin~ l!oil fertility, whil'h ha"e been enumerated in this circuhr. He should use all precautions that his soils do not bel'ome infertile because of improrer ro· tations, poor dminave, lnek of l'are in the p::eparation of the !!eed bed and the cultil'ation of the crop, insufficient plant food, the de,·elopment of acid l'onditions, or dcfieiencv in humus content. Observ:n~ all these preeautions there is no need of Iowa soils wcarin~t out; and neglecting them there is no doubt but that such a wearing out will occur.